These studies will help clarify the manner in which aggressive experience influences future behavior by investigating neuroendocrine mechanisms involved. This research will incorporate hormonal, neural and behavioral approaches to the study of female aggression, which has been little studied outside of maternal aggression. Specifically, these studies will investigate how experience and hormones interact to alter aggressive behavior and theses differences in aggression are related to differences in neuronal activity. First, I will demonstrate plasticity in aggression in female Peromyscus californicus and their ability to win encounters both due to previous experience and across reproductive states. Second, I will determine whether estrogen, progesterone, testosterone or corticosterone is associated with baseline levels and/or changes in aggression, as well as investigate possible bidirectional relationships between these hormones and aggression. Third, I will investigate several brain areas known to be involved in aggression, and determine the amount of c-fos activity associated with aggression and changes in aggression due to previous experience and/or reproductive state. These areas will include the medial preoptic area-anterior hypothalamus, lateral septa, bed nucleus of the stria terminals, medial and corticomedial amygdala, and the preoptic, ventromedial and peripeduncular nuclei. Together, these studies will help to identify whether (1) baseline hormone levels; (2) change in hormone levels; (3) amount of neural activity; and/or (4) changes in location of neuronal activity can explain how information from previous aggressive encounters can alter outcomes of future aggressive encounters.